Recent events have emphasized the need for reliable communication systems during emergencies in underground and hazardous work areas such as coal mines. During a mine disaster, the current voice and data communication system usually fails or is shut down to prevent an explosion so the conditions of personnel, environment and equipment in the area is unknown which complicate recovery efforts. Past mining accidents have demonstrated that current communication systems are not sufficient to provide the support required to effectively handle evacuation and rescue operations. The 2006 MINER Act amends the Federal Mine safety and Health Act of 1977 stating underground coal mine operators must provide for post accident communication between underground and surface personnel via a wireless two-way medium within three years. The 2006 MINER Act also requires an electronic tracking system in order for surface personnel to determine the location of any person trapped underground. Robust and reliable communications are critical for both normal operations and in the event of an emergency. The National Institute for Occupational Safety and Health (NIOSH) released a solicitation in late 2006 for an underground communication system that is highly reliable and provides in-mine and mine-to-surface voice and data communications based on wireless mesh network technology as part of an underground communications system.
Methods of wireless communications above ground are not effective in mines, tunnels and other underground facilities due to the environment and limited radio wave propagation. Prior art communication systems for underground use include Leaky Feeder systems, wireline repeater systems, wireless repeaters systems and through-earth radio systems.
Leaky Feeder systems consist of one or more base stations above ground with leaky feeder coax cable below ground that distributes an RF signal between above ground base stations and mobile radios below ground to provide voice and data communications to personnel. The Leaky Feeder system has built in RF amplifiers at regular intervals in the coax cable to extend the distance that can be covered by the system. The distance from the Leaky Feeder cable to a mobile radio in an underground mine is limited to 50-150 feet depending on location of the RF amplifier and the environment. The Leaky Feeder base stations are connected to an Operations Center above ground. In the event of a disaster, Leaky Feeder coax cable is often damaged underground which prevents communications from the base stations to the mobile radios. Unless the Leaky Feeder system is approved by Mine Safety and Health Administration (MSHA) for operating in a hazardous environment, it is automatically shut down during an emergency.
Wireline RF repeater communication systems operate similarly to Leaky Feeder systems except a low loss coax cable interconnects to a series of underground RF repeaters which communicate with the mobile radios. In the event of a disaster, the wired system is often damaged underground that prevents communication. Wireline RF Repeater systems have been replaced by Leaky Feeder system because the Leaky Feeder is more cost effective.
Wireless repeater systems are similar to wireline repeater systems except the low loss coax cable is replaced with wireless links to underground RF repeaters which can improve the link reliability in event of a disaster. However, all communications are still controlled by the aboveground base stations. If that connection is missing, underground communications are lost since the below ground units are simply repeaters.
Through-earth radio systems that operate at very low frequency have been experimented with but have not been proven to be reliable and cost effective.
No single prior art for use in a underground mine or hazardous area has the ability to provide a reliable voice and data communications network, reform and provide redundant paths, interface to wired communications, and provide location information of personnel in the hazardous area as specified by 2006 Miner Act and NIOSH.
Existing wireless mesh networking with an IP network layer include the IEEE 802.11 standard and IEEE 802.15.1 (Bluetooth). Both standard technologies perform an excellent role for which they were designed. However, the link protocols do not form the desired network topology to achieve an optimal mesh deployment solution. The use of the 2.4 GHz unlicensed RF spectrum band allows fast deployment anywhere, anytime, but may not meet reliability requirements for critical communications. These bands are not dedicated and can be jammed by other commercial users. In addition, operation in the 2.4 GHz band provides a higher cost solution due to less RF coverage per FMN in an underground environment.
Thus, there is a need to develop a cost effective wireless communication system with reliable underground voice and data network. An intrinsically safe low cost wireless ad hoc mesh network with battery backup and approval for deployment in hazardous environments can achieve such a solution. A complete wireless communication network includes mesh network routers that operate below ground with access points above ground that connect to external networks to provide dispatch, collaborative detection, location, assessment, and tracking during emergency events as well as normal operation. Nevertheless, there are several technical issues with current wireless networks which need to be addressed: high reliability network architecture, placement of fixed wireless nodes for reliable coverage, scalability to support a large network, selection of radio RF frequency, and waveform protocol for communication and tracking of personnel.